Charged particles have been used in the field of radiation therapy for cancer for more than 50 years. In order to create a clinically useful dose distribution that conforms to the shape of the target volume within the patient, a number of beam shaping and modulating materials are interposed between the particle accelerator and the patient. A proton beam has a significant clinical advantage over conventional high energy x-ray beams which attenuate exponentially in tissue. The physics of the energy deposition is advantageous and different for protons compared to high energy x-rays (photons).
A proton beam delivers a small entrance dose, then delivers a large dose as the protons stop in the tissue. This large deposition of dose at the end of the tissue penetration range of the protons is called a Bragg peak, after the physicist who discovered the effect. FIG. 1 shows the Bragg peak from an unmodulated beam, as well as a spread out Bragg peak and the series of individual Bragg peaks that add together to make the spread out Bragg peak.
The beam, emerging from the particle accelerator, is shaped by inserting devices and materials into the beam. One objective of shaping the beam is to deliver a uniform dose of radiation throughout the volume of a target, such as a tumor in a patient's body. The range (i.e. the depth of beam penetration into the tissue) needs to be modulated to ensure that a uniform or other predetermined dose of radiation is delivered between the proximal and the distal surfaces of the target. (As used herein, the terms “proximal” and “distal” are used with respect to the beam path. The term “proximal” specifically refers to the area of entry of a beam into a target.) Furthermore, the beam needs to be spread out laterally in order to treat large tumors. (As used herein, the terms “lateral” refers to any direction substantially perpendicular to the beam path.) The beam is manipulated and shaped by a series of scatterers and apertures.
In a beam shaping system, the beam is first directed at a first scatterer/range modulator, which scatters the proton beam through an angle wide enough to treat a therapy field of about 20–30 cm. Following scattering and range modulation by the first scatterer, the beam is directed to a compensated second scatterer. The purpose of this element is to flatten the cross section of the beam emerging from the first scatterer. This allows the Bragg peak to be planar and uniform in intensity at the isocenter distance. FIG. 2 shows a compensated second scatterer that is comprised of high Z and low Z materials with shapes that match the scattering property of the high Z material with the absorption properties of the low Z material in order to provide a flat, uniform broad beam.
The third element of the beam shaping system is a range matching bolus. This is typically a thick cylinder of acrylic plastic into which the inverse of the 3-dimensional shape of the distal surface of the target volume has been machined. This element also includes a correction for the profile of the external surface of the patient from the beam direction and a correction for the inhomogenieties such as bone or air in the path. Most tissue is substantially equivalent to water, but corrections for these different materials can be calculated from the CT image data set. The resulting three dimensional structure is placed in the beam path to ensure that the Bragg peak conforms to the distal surface of the target, resulting in minimum dose to critical structures located beyond the target volume.
The fourth element of the beam shaping system shapes the beam laterally to match the shape of the target volume as seen from the direction of the beam's origin by using apertures made specifically for that treatment. This is usually accomplished by machining a profiled aperture into a thick piece of brass or other high Z material and placing it in close proximity to the patient. The beam is limited in lateral extent by this element and therefore conforms to the shape of the target volume.